首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1472465篇
  免费   134036篇
  国内免费   1614篇
  2021年   18163篇
  2019年   16447篇
  2018年   18920篇
  2017年   17587篇
  2016年   29145篇
  2015年   43457篇
  2014年   51524篇
  2013年   78100篇
  2012年   41751篇
  2011年   31098篇
  2010年   45883篇
  2009年   46599篇
  2008年   28761篇
  2007年   27045篇
  2006年   31254篇
  2005年   32080篇
  2004年   31241篇
  2003年   28581篇
  2002年   26653篇
  2001年   44171篇
  2000年   41464篇
  1999年   38842篇
  1998年   26522篇
  1997年   26435篇
  1996年   25611篇
  1995年   23765篇
  1994年   23443篇
  1993年   22656篇
  1992年   32295篇
  1991年   30432篇
  1990年   28873篇
  1989年   29378篇
  1988年   26841篇
  1987年   25153篇
  1986年   23889篇
  1985年   25556篇
  1984年   24699篇
  1983年   21606篇
  1982年   21339篇
  1981年   20517篇
  1980年   19163篇
  1979年   20584篇
  1978年   18987篇
  1977年   18230篇
  1976年   17090篇
  1975年   16667篇
  1974年   17272篇
  1973年   17586篇
  1972年   15032篇
  1971年   13667篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Methanobacterium thermoautotrophicum, an archaebacterium, possesses the first and last enzymes of the diaminopimelic acid pathway for lysine biosynthesis, dihydrodipicolinate synthase, and diaminopimelate decarboxylase. It does not have saccharopine dehydrogenase, the last enzyme of the aminoadipate pathway for lysine biosynthesis. The dihydrodipicolinate synthase is inhibited but not repressed by lysine. We conclude that this microbe uses the diaminopimelate pathway for synthesis of lysine.Deceased.  相似文献   
993.
Addition of a thermostable cytoplasmic fraction leads to the uncoupling of oxidative phosphorylation of the mitochondria. In hyperthyrosis such an effect manifests itself more powerfully than in the control. Addition of the thermostable cytoplasmic fraction induces electrogenic phosphate transport via the mitochondrial membrane. In hyperthyrosis, the activity of the thermostable inducer of phosphate transport in the cytoplasm increases. The functioning of the phosphate cycle may be the cause of the uncoupling of oxidative phosphorylation of the mitochondria during the disease in question.  相似文献   
994.
995.
Additive reagents have been investigated to improve the stability of methanolic Wright's stain. The addition of ammonium halides, monoalkyiamine hydrochlorides, dialkylamine hydrochlorides or trialkylamine hydrochlorides to methanolic Wright's stain was found to enhance the stability of stain components in methanol. No change in performance is observed with these additives present. Random precipitation in the stain solution was still observed with the addition of ammonium halides and monoalkyiamine hydrochlorides. No precipitation was found in stain solutions containing hydrochlorides of most dialkylamines and trialkylamines. Of the compounds evaluated, 0.6% diethylamine hydrochloride added to methanolic stain solutions produced the most desirable overall results. Mechanisms of stabilization and precipitation in these stain solutions are proposed, Essentially, separation of the thiazine-eosinate ion pair through interaction with an appropriate additive increases stain stability. The solubilities of thiazine-eosinate or additive cation-eosinate ion pairs in methanol determine the formation of precipitate in such stain solutions.  相似文献   
996.
Two general models for the transspecific evolution of butterfly colour patterns are advanced: directional selection acting equally on both sexes, and disruptive selection involving periods of polymorphism. To consider possible outcomes of me latter process, a morphism notation based on an integrated classification for polymorphism and sexual dimorphism is developed. This notation is used to examine the properties of all morphism transformations possible from the minimal expressions of the nine morphism categories, as reached through defined minimum step changes. The significance of such pathway models is analysed in terms of general properties of butterfly polymorphism. The potential use of pathway models in evolutionary studies is briefly discussed, mainly with respect to phylogenetics, and ideas on the evolution of genetic dominance.  相似文献   
997.
998.
The ilvI and ilvH gene products were identified physically by electrophoretic analysis of in vivo-labelled polypeptides produced in minicells from plasmids carrying the wild-type ilvIH operon of Escherichia coli K-12 and derivatives of it. An analysis of the distribution of methionine residues in the amino-terminal portion of micro-quantities of the ilvI product eluted from gel showed that the translational start of the ilvI gene is the promoter-proximal one of three putative methionine codons predicted from the DNA sequence.  相似文献   
999.
  1. Assemblages of insect herbivores are structured by plant traits such as nutrient content, secondary metabolites, physical traits, and phenology. Many of these traits are phylogenetically conserved, implying a decrease in trait similarity with increasing phylogenetic distance of the host plant taxa. Thus, a metric of phylogenetic distances and relationships can be considered a proxy for phylogenetically conserved plant traits and used to predict variation in herbivorous insect assemblages among co‐occurring plant species.
  2. Using a Holarctic dataset of exposed‐feeding and shelter‐building caterpillars, we aimed at showing how phylogenetic relationships among host plants explain compositional changes and characteristics of herbivore assemblages.
  3. Our plant–caterpillar network data derived from plot‐based samplings at three different continents included >28,000 individual caterpillar–plant interactions. We tested whether increasing phylogenetic distance of the host plants leads to a decrease in caterpillar assemblage overlap. We further investigated to what degree phylogenetic isolation of a host tree species within the local community explains abundance, density, richness, and mean specialization of its associated caterpillar assemblage.
  4. The overlap of caterpillar assemblages decreased with increasing phylogenetic distance among the host tree species. Phylogenetic isolation of a host plant within the local plant community was correlated with lower richness and mean specialization of the associated caterpillar assemblages. Phylogenetic isolation had no effect on caterpillar abundance or density. The effects of plant phylogeny were consistent across exposed‐feeding and shelter‐building caterpillars.
  5. Our study reveals that distance metrics obtained from host plant phylogeny are useful predictors to explain compositional turnover among hosts and host‐specific variations in richness and mean specialization of associated insect herbivore assemblages in temperate broadleaf forests. As phylogenetic information of plant communities is becoming increasingly available, further large‐scale studies are needed to investigate to what degree plant phylogeny structures herbivore assemblages in other biomes and ecosystems.
  相似文献   
1000.
S Mikamo 《Human cell》1992,5(3):256-266
Inflammatory cytokines are able to facilitate the homing of transferred lymphocytes, tumor cell lysis through induction of adhesion molecules, also able to reduce tumor cell susceptibility to LAK cells by increasing tumor cell class I antigen. Investigation with 12 cell lines suggested that promotion of lysis by ICAM-1 was more responsible than protection by (allogeneic) class I Ags (Fig. 1). PBMC were cultured in anti-CD3 coated flasks with rIL-2. CD3+ cells dominated until day 7, decreased thereafter with CD4+. CD8+ and CD16+ increased (Fig. 2). Strong cytotoxicity obtained in some cultures correlated well with CD16+, contributing exclusively among several variables to the activity estimation in multiple regression analysis (Fig. 4). Among 6 cases, in which 2 or more cycles of transfer was done, 1 was prophylaxis of recurrence, in 2 of 3 advanced metastasis cases in which cells were transferred as BRM in the course of chemotherapy, survival of half a year was obtained in good QOL with suppressed disease and adequate level of PBL number. In 2 other cases, inflammation eliciting local treatments were combined. In the case 4, three large liver metastasis from colon cancer which resisted topical ethanol injection and chemotherapy, responded to the transfer with reduced lesions to 1/8 (Fig. 8). In the case 5, abdominal metastasis from colon cancer were removed, liver metastasis were injected of ethanol, and cells were transferred. Responses were obtained to immunotherapy in a certain degree, while never to any chemotherapy.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号